Seismic data is used in many scientific fields to monitor underground events in subterranean rock formations. In order to investigate these underground events, micro-earthquakes, also known as microseisms, are detected and monitored. Like earthquakes, microseisms emit elastic waves—compressional (“p-waves”) and shear (“s-waves”), but they occur at much higher frequencies than those of earthquakes and generally fall within the acoustic frequency range of 200 Hz to more than 2000 Hz. Standard microseismic analysis techniques locate the sources of the microseismic activity by fluid injection or hydraulic fracturing. In many gas fields, permeability is too low to effectively produce gas in economic quantities. Hydraulic fracturing addresses this problem by intentionally creating fractures in the gas fields that provide conduits to enhance gas flow. Fluid is pumped into wells at sufficient pressure to fracture the rock. The fluid also transports a propping agent (also known as “proppant”) into the fracture. The proppant, usually sand or ceramic pellets, settles in the fractures and helps keep the fracture open when the fracturing operation ceases. Production of gas is accelerated as a result of improved capability for flow within the reservoir. Similarly, water flooding of largely expended oil fields seeks to push oil to other wells where it is produced. Fractures are often created in this process that direct the oil in a potentially unknown direction. In this process, water, or possibly steam, is used to increase pressure and/or temperature to displace the oil to a more favorable production location.
Microseismic detection is often utilized in conjunction with hydraulic fracturing or water flooding techniques to map created fractures. A hydraulic fracture induces an increase in the formation stress proportional to the net fracturing pressure as well as an increase in pore pressure due to fracturing fluid leak off. Large tensile stresses are formed ahead of the crack tip, which creates large amounts of shear stress. Both mechanisms, pore pressure increase and formation stress increase, affect the stability of planes of weakness (such as natural fractures and bedding planes) surrounding the hydraulic fracture and cause them to undergo shear slippage. It is these shear slippages that are analogous to small earthquakes along faults.
Microseisms are detected with multiple receivers (transducers) deployed on a wireline array in one or more offset well bores. With the receivers deployed in several wells, the microseism locations can be triangulated as is done in earthquake detection. Triangulation is accomplished by determining the arrival times of the various p- and s-waves, and using formation velocities to find the best-fit location of the microseisms. However, multiple offset wells are not usually available. With only a single nearby offset observation well, a multi-level vertical array of receivers is used to locate the microseisms. Data is then transferred to the surface for subsequent processing to yield a map of the hydraulic fracture geometry and azimuth. Once the microseisms are located, the actual fracture is interpreted within the envelope of microseisms mapped. However, the precise length, direction, and height of the created fractures will not be obtainable unless the microseismic events are accurately detected from beginning to end.